The following characteristics have conventionally been required in various substrate glasses for a display, particularly substrate glasses having a metal or an oxide thin film formed on the surface thereof.
(1) In the case where an alkali metal oxide is contained, alkali metal ions diffuse in the thin film, resulting in deterioration of film characteristics. For this reason, alkali metal ions are not substantially contained.
(2) A strain point is high such that deformation of a glass and shrinkage (thermal shrinkage) due to structure stabilization of a glass can be minimized when exposed to high temperature in a thin film formation step.
(3) A glass has sufficient chemical durability to various chemicals used in semiconductor formation. Particularly, the glass has durability to buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiOx or SiNX, a medicinal solution containing hydrochloric acid used for etching ITO, various acids (nitric acid, sulfuric acid and the like) used for etching an metal electrode, and an alkaline of a resist stripping solution.
(4) Defects (bubble, striae, inclusion, pit, flaw and the like) are not present in the inside and on the surface.
In addition to the above requirements, the recent years are under following situations.
(5) Reduction in weight of a display is required, and a glass itself is required to have small density.
(6) Reduction in weight of a display is required, and decrease in thickness of a substrate glass is desired.
(7) In addition to the conventional amorphous silicon (a-Si) type liquid crystal display, a polycrystal silicon (p-Si) type liquid crystal display in which heat treatment temperature is slightly high has began to be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.).
(8) A glass having small average thermal expansion coefficient is required in order to improve productivity and increasing thermal shock resistance by increasing a temperature-rising rate in a heat treatment for liquid display preparation.
On the other hand, dry etching progresses and requirement to BHF resistance is becoming weakened. Many glasses conventionally used are glasses containing 6 to 10 mol % of B2O3 in order to improve BHF resistance. However, B2O3 has the tendency to decrease a strain point. The following glasses are exemplified as an alkali-free glass that does not contain B2O3 or contains B2O3 in small amount.
Patent Document 1 discloses SiO2-Al2O3-Sro glass that does not contain B2O3. However, a temperature required for melting is high, and this causes difficulty in production.
Patent Document 2 discloses SiO2-Al2O3-Sro crystallized glass that does not contain B2O3. However, a temperature required for melting is high, and this causes difficulty in production.
Patent Document 3 discloses a glass containing B2O3 in an amount of from 0 to 3% by weight. However, an average thermal expansion coefficient at from 50 to 300° C. exceeds 40×10−7/° C.
Patent Document 4 discloses a glass containing B2O3 in an amount of from 0 to 5 mol %. However, an average thermal expansion coefficient at from 50 to 300° C. exceeds 50×10−7/° C.
Patent Document 5 discloses a glass containing B2O3 in an amount of from 0 to 5 mol %. However, thermal expansion is large and density is also large.
To solve the problems in the glasses described in Patent Documents 1 to 5, an alkali-free glass described in Patent Document 6 is proposed. The alkali-free glass described in Patent Document 6 has high strain point, can be formed by a float process, and is considered to be suitable for uses such as a display substrate and a photomask substrate.
A production method of high quality p-Si TFT includes a solid phase crystallization method. However, to carry out the method, it is required to further increase a strain point.
On the other hand, to comply with the demand in glass production process, particularly melting and forming, a glass is required to have lower viscosity, particularly lower temperature T4 at which a glass viscosity is 104 dPa·s.